In wireless communication systems, knowledge of antenna correlations at a transmitter, for example, a base station, is important for beamforming transmissions (also referred to as precoding) that deliver more power to a targeted user while minimizing interference on other users. Precoding operations can be in the context of single-user multiple input multiple output (SU-MIMO) or multi-user MIMO (MU-MIMO), where two or more users are served by a single base station. Precoding operations may also be in the context of SU/MU-MIMO users served by coordinated multiple points (CoMP) transmission. Due to the large number of antennas involved in such transmissions (8-Tx eNB, CoMP, etc.), it is desirable that the base station flexibly configure the computation, parameterization and reporting of antenna correlation to reduce feedback overhead.
In research publications, complete channel knowledge at the transmitter is assumed, which relates to antenna correlation since it is derived from the channel. However, the practical mechanism of how to obtain such knowledge is unaddressed and is generally left to the air interface design. In existing 4G air interface specifications such as the Third Generation Partnership Project Long Term Evolution (3GPP LTE) and IEEE 802.16e, there is no feedback of any kind of antenna correlation. In IEEE 802.16m that is under standardization, the practice of antenna correlation reporting is limited to the correlation matrix computed across all transmit antennas in a non-configurable fashion. The dimension of a correlation matrix depends on the number of antennas involved in the transmission. The number of unique entries of the correlation matrix also grows in proportion to the square of the dimensionality, which is the number of antennas for which their correlations need to be computed. The increasing number of entries of the correlation matrix will incur a significant overhead causing a feedback bottleneck. A configurable method that can parameterize the antenna correlation more efficiently and take advantage of any redundancy introduced by antenna array configurations, such as long term channel behavior, is desirable.
Further, CoMP approaches can potentially perform joint precoding and transmission from a super-set of antennas corresponding to all the coordinating points. These coordinating points may be individual cells or sectors, remote radio heads (RRHs), relays, HeNBs, pico-cells etc. More generally the coordinating points may be different transmission points in a heterogeneous network. In such cases, a transmitter may prefer to obtain feedback on a large number of antennas corresponding to these transmission points.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.